Designing for UI density goes beyond the visual aspects of an interface. It includes all the implicit and explicit design decisions we make, and all the information we choose to show on the screen. It includes all time and the actions a user takes to get something valuable out of the software.

So, finally, a concrete definition of UI density: UI density is the value a user gets from the interface divided by the time and space the interface occupies.

Speed, usability, consistency, predictability, information richness, and functionality all play an important role in this equation. By taking account of all these aspects, we can understand why some interfaces succeed and others fail. And by designing for density, we can help people get more value out of the software we build.